As integrated circuits become smaller and faster, the critical dimensions (CD's) of the devices and interconnections also must decrease. As these CD's get closer to the resolution limits of optical lithography and microscopy measurement techniques, great care must be taken to eliminate all possible sources of measurement error in order to obtain accurate and reproducible CD's. One nearly universally used measurement technique is Scanning Electron Microscopy (SEM), which utilizes highly focused beams of electrons impinging on the sample and measures the yield of secondary emitted electrons. SEM is the most widely used tool for Very Large Scale Integration (VLSI) measurement and morphology analysis, due to its high resolution and relative ease of use.
FIG. 1a depicts an SEM system, showing electron beam (80) from electron source (100) impinging onto sample (90), and the acceleration (102), focusing (104), scanning (105) and detection (106) electronics. FIG. 1b shows a typical electron emission energy spectrum resulting from the incident electron beam of an SEM. The highest energy peak (108) results from the backscattered electrons, which have energies close to that of the incident beam, and which have undergone only elastic collisions with the target atoms. Peaks 110 seen at intermediate energies are the Auger electrons emitted due to relaxation of electrons between atomic energy levels. The lowest energy emitted electrons (112), produced by inelastic collisions between the primary beam and the inner shell electrons of the sample, are known as the secondary electrons and are generally the most useful for morphology studies in VLSI. This is due in part to the extremely short escape depth (less than about 50 Angstroms) of secondary electrons, which yields high surface sensitivity. In addition, since the incident electron beam undergoes beam broadening due to multiple collisions as it penetrates into the sample, the backscattered electrons originating from deeper into the sample reflect this broadening with degraded point-to-point resolution. The lower energy secondary electrons which escape the sample originate from the surface region above the penetration depth where beam broadening becomes influential, and therefore yield higher point-to-point resolution than evidenced by backscattered electrons.
The detected electron current, typically chosen to be the secondary electron current as described above, is used to intensity modulate the z-axis of a Cathode Ray Tube (CRT). An image of the sample surface is produced by raster scanning the CRT screen and the electron beam of the SEM.
The contrast of the image depends on variations in the electron flux arriving at the detector, and is related to the yield of emitted electrons per incident electron. The yield is dependent on both the work function of the material and the surface curvature. These factors allow the SEM to distinguish between materials such as photoresist, metal, oxide, and silicon, and also to distinguish surfaces which differ in slope. Thus, CD's of patterned and/or etched lines and gaps can be measured.
Two important factors affecting the accuracy and reproducibility of SEM measurements of CD's in photoresist layers are resist shrinkage and charging effects. Resist shrinkage can occur due to such factors as elevated temperatures or evaporation, crosslinking of the polymer chains, purely thermal reactions, diffusion of acid and subsequent deprotection, or solvent loss.
Charging effects are also a cause of unstable and inaccurate SEM measurement results. When the number of emitted secondary electrons is different from the number of incident electrons, the surface scanned by the beam acquires excess charge, which may be retained, particularly in the case of exposed insulating surfaces. This will cause the incident beam trajectory to be disturbed, and will therefore degrade the image and destabilize the measurements. Additionally, charging of the surface may contribute to resist shrinkage, by enhancing causative factors such as polymer cross-linking.
Present technology utilizes 193 nm photoresist for patterning in the 130 nm–100 nm range. Standard 193 nm resist is generally Argon-Fluoride resist (ARF). 193 resist is known to shrink substantially when exposed to an electron beam. Consequently, it yields poor measurement precision if no correction is used. A method for stabilizing CD-SEM measurements on ArF resist layers would be of great utility in current semiconductor manufacturing technology.